Fuel Why does the fuel pump prime and then stop?

[evintho]

Little back story. My 35 year old engine control wiring harness was toast so I went with a Painless harness. It does away with a lot of the superfluous crap that's not needed.

https://www.summitracing.com/parts/prf-60510?srsltid=AfmBOorr5PZ4VJ8SF2f_Vs_Em8YVra1KZ_I_5ggDCRO9m5cg-SC2KPCr_pE

So, I wired the starter relay wire as per instructions: Connect wire to the B+ side of starter relay OR to the positive side of battery. This wire provides constant battery 12v power to the computer and relays.
Well.......I connected it to the POS side of the relay (which gets constant 12v power) and when I turn the key to ON, the fuel pump primes but doesn't stop! So, what makes the fuel pump stop priming? Is it because it reaches 39 psi in the lines? Why does my FP continue to run? Oh yeah, '93 5.0.

 

[Noobz347]

From @jrichker 's checklist:

Clue – listen for the fuel pump to prime when you first turn the ignition switch on. It should run for 2-4 seconds and shut off. To trick the fuel pump into running, find the EEC test connector and jump the connector in the Upper RH corner to ground. The EEC connector is near the wiper motor and LH hood hinge.

If the relay & inertia switch are OK, you will have power to the pump. Check fuel pressure – remove the cap from the Schrader valve behind the alternator and depress the core. Fuel should squirt out, catch it in a rag. Beware of fire hazard when you do this. In a pinch, you can use a tire pressure gauge to measure the fuel pressure. It may not be completely accurate, but you will have some clue as to how much pressure you have. If you have any doubts about having sufficient fuel flow/pressure, rent a fuel pressure test gauge from the auto parts store. That will tell you for sure if you have adequate fuel pressure.


4.) No fuel pressure, possible failed items in order of their probability:
A.) Tripped inertia switch – Coupe & hatch cars hide it under the plastic trim covering the driver's side taillight. Use the voltmeter or test light to make sure you have power to both sides of the switch
B.) Fuel pump power relay – located under the driver’s seat in most stangs built before 92. On 92 and later model cars it is located below the Mass Air Flow meter. Look for 12 volts at the Pink/Black wire on the fuel pump relay.
C.) Clogged fuel filter
D.) Failed fuel pump
E.) 86-90 models only: Blown fuse link in wiring harness. Look for 12 volts at the Orange/Lt Blue wire on the fuel pump relay.
91-93 models only Blown fuse link in wiring harness. Look for 12 volts at the Pink/Black wire on the fuel pump relay.
The fuse links for all model years 86-93 live in the wiring harness near the starter solenoid.

 

[Mustang5L5]

So, what makes the fuel pump stop priming? Is it because it reaches 39 psi in the lines? Why does my FP continue to run? Oh yeah, '93 5.0.

It's timed off the ECU. When you turn the key to on, it cycles for 2-4 seconds and then turns off as it receives no signal that the engine is running. When you start the car, the ECU now receives that signal so the pump runs continuous.

As shown above, you can jump a wire out to test the pump and run it continuous. Other reason for continuous fuel pump running is ECU issues. It's possible to damage the ECU if you put voltage on pin 46 of a manual trans ECU. Turn key to on and check for voltage from pin 46 to ground. Shoyld be 0 volts

 

[evintho]

Thanks! That's exactly what I needed to know. I was on the phone with Painless tech today and he gave me a few things to try.